1. Field of the Invention
The present invention relates to a method of compensating for a rotation angle detected with a magnetoresistive element typified by a giant magnetoresistive (GMR) element and to an angle sensor using the compensation method. More particularly, the present invention relates to a method of calculating a compensation value for an angle detecting sensor, capable of improving the detection accuracy of a detected output angle even if error signals including a phase error and/or a distortion error are included in signals output from a magnetoresistive element, and to an angle detecting sensor using the method.
2. Description of the Related Art
An output angle of, for example, a steering wheel of an automobile is detected with a wheel that rotates in synchronization with a rotating member, such as the steering shaft, and with an angle detecting sensor. The sensor unit in the angle detecting sensor adopts magnetoresistive elements that sense magnetism to output an output signal. Related arts of angle detecting sensors using the magnetoresistive elements are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 2002-303536, 2000-35470, 2003-106866, and 2003-66127 (Patent Documents 1 to 4).
FIG. 16 is a plan view showing an example of the structure of an angle detecting sensor 100. The angle detecting sensor 100 has a wheel 102 that rotates about the center O of rotation and a package 101 provided inside the wheel 102.
The package 101 includes four chip substrates (wafers) K1, K2, K3, and K4 that are arranged symmetrically with respect the center O of rotation (arranged at positions shifted from each other by 90° around the center O of rotation). Each of the chip substrates K1 to K4 includes two GMR elements (denoted by G1 to G8) serving as magnetoresistive elements. Each GMR element basically has a structure in which an exchange bias layer (antiferromagnetic layer), a fixed layer (pinned layer), a non-magnetic layer, and a free layer (free magnetic layer) are layered.
Specifically, the chip substrate K1 includes the GMR elements G1 and G2, the chip substrate K2 includes the GMR elements G3 and G4, the chip substrate K3 includes the GMR elements G5 and G6, and the chip substrate K4 includes the GMR elements G7 and G8. Referring to FIG. 16, the GMR elements G1 and G4 connected in series to each other are connected in parallel to the GMR elements G3 and G2 connected in series to each other to form a first bridge circuit. Similarly, the GMR elements G5 and G8 connected in series to each other are connected in parallel to the GMR elements G7 and G6 connected in series to each other to form a second bridge circuit.
Magnets M1 and M2 are fixed inside the wheel 102. The magnet M1 is fixed in a state in which the north pole is directed to the center O of rotation and the magnet M2 is fixed in a state in which the south pole is directed to the center O of rotation. A predetermined external magnetic field H is generated between the magnets M1 and M2.
Rotation of the rotating member, which is an object to be measured, to rotate the wheel 102 causes the magnets M1 and M2 to circle around the package 101. At this moment, the direction of magnetization of the free layer of each of the GMR elements G1 to G8 is varied in accordance with the external magnetic field H. This variation varies the resistance of each of the GMR elements G1 to G8 in accordance with an angle between the direction of magnetization of the free layer and the direction of magnetization of the fixed layer. As a result, +sin and −sin signals are output from the first bridge circuit and, simultaneously, +cos and −cos signals are output from the second bridge circuit. The phases of the ±cos signals are shifted from those of the ±sin signals output from the first bridge circuit by 90°.
A controller differentially amplifies the +sin and ±sin signals, among the four signals, to generate a SIN signal (sinusoidal signal) and differentially amplifies the +cos and −cos signals to generate a COS signal (cosine signal). Then, the controller calculates a tangent (tan) from the SIN signal (sinusoidal signal) and the COS signal (cosine signal) and calculates an arctangent (arctan) in order to detect an output angle of the rotating member.
In order to detect the rotation angle of the rotating member with high accuracy in the angle detecting sensor 100, it is necessary to accurately maintain the phase difference 90° between the sinusoidal signal and the cosine signal. To this end, it is necessary to mount the chip substrates such that the directions h of magnetization of the adjacent chip substrates are accurately shifted from each other by 90° because the fixed layers of the two GMR elements provided on the same chip substrate have the same direction h of magnetization. For example, when the direction h of magnetization of the chip substrate K1 is +Y direction, the adjacent chip substrates are arranged such that the direction h of magnetization of the chip substrate K2 becomes −Y direction, the direction h of magnetization of the chip substrate K3 becomes +X direction, and the direction h of magnetization of the chip substrate K4 becomes −X direction.
However, since it is not possible to visually confirm the directions h of magnetization of the fixed layers of the GMR elements G1 to G8, it is difficult to mount the chip substrates K1 to K4 on the package 101 such that the directions h of magnetization of the chip substrates are accurately shifted from each other by 90°. If it is not possible to accurately set the shift to 90°, a phase error a occurs and the phase difference becomes 90°±α. Consequently, there is a problem in that the rotation angle (output angle) of the rotating member cannot be accurately detected.
When the chip substrates are accurately cut out and the directions h of magnetization of the GMR elements G1 to G8 are almost in parallel with sides of the chip substrates, an apparatus, for example, an image recognition apparatus, which compensates for the mounting angle can be used to mount the chip substrates on the package 101 such that the positions of the chip substrates are shifted from each other by 90° in order to accurately shift the directions h of magnetization by 90°. However, there are problems in that it is likely to increase the manufacturing cost of the chip substrates and that the assembly process becomes complicated to increase the assembly time and the assembly cost.
Although it is desirable that an output angle φ be exactly proportional to an input angle (the rotation angle of the magnets) θ at which the wheel 102 rotates in the angle detecting sensor 100, the actual output angle φ ordinarily has a distortion error β resulting from superimposition of the sinusoidal signal on a straight line, which is a linear function, and there are cases in which the output angle φ is not exactly proportional to the input angle θ (refer to FIG. 6).
Such a distortion error β can be caused by resistance distortion specific to the GMR element. If the four signal waveforms output from the angle detecting sensor 100 include such distortion errors, the distortion errors also occur in the SIN and COS signals and the calculation of the tangent and the arctangent is also affected by the distortion errors. According, there is a problem in that it is not possible to improve the accuracy of the output angle φ detected by the angle detecting sensor 100.
The phase error α and the distortion error β can be approximated with a predetermined function, and the accuracy of the angle output can be greatly improved if the approximation function can be used to individually compensate for the output angle φ output from the angle detecting sensor 100. However, it is not easy to calculate compensation factors forming the predetermined function and there is no description of how to calculate the compensation factors in Patent Documents 1 to 4.